1. Field of the Invention
The present invention relates to a transistor switch device capable of operation with a reduced voltage drop at the collector of the main power transistor.
2. Description of the Prior Art
High-power transistors, in comparison with thyristors, have a self turn-off function but cause a high collector voltage drop in the high-voltage element during conduction, resulting in a large loss. In a Darlington transistor circuit, the loss is especially large, amounting to two to three times that in a thyristor circuit. In applications where large power is controlled or converted, the Darlington transistor circuit has been considered impracticable in view of its need for considerable cooling and its low current utilizing efficiency.
With respect to FIGS. 1(a) and 1(b), there are shown circuit diagrams of prior art power transistor switch devices. FIG. 1(a) shows a conventional Darlington circuit, comprising power sources 100a and 100b, a load 200, main power transistors 1a and 1b, and auxiliary transistors 2a and 2b connected between the collectors and the bases, respectively, of the main power transistors 1a and 1b. In this Darlington circuit, the necessary collector-emitter voltage V.sub.CE.sbsb.2 (ON) of the auxiliary transistor 2 in the ON state is supplied from the collector-base voltage V.sub.CB.sbsb.1 (ON) of the main transistor 1 in the ON state and hence the collector-emitter voltage V.sub.CE.sbsb.1 (ON) of the main transistor 1 in the ON state becomes large causing the collector loss to be increased during power supply. This has lowered the current utilizing efficiency and made cooling difficult. For these reasons, the Darlington transistor circuit as in FIG. 1(a) is not practical for use as a large-capacity power switch device.
FIG. 1(b) shows another power transistor switch device in which the voltage drop at the collector of the main transistor 1 in the ON state as well as loss can be reduced. However, a rather large amount of power is required from power sources B.sub.1 and B.sub.2 to drive the bases thereof. The power can be reduced but at the sacrifice of greater collector voltage drop and loss in the main power transistor 1.